Chapter 6: Arrays

Report
Chapter 6: Arrays
Presentation slides for
Java Software Solutions
for AP* Computer Science
by John Lewis, William Loftus, and Cara Cocking
Java Software Solutions is published by Addison-Wesley
Presentation slides are copyright 2002 by John Lewis, William Loftus, and Cara Cocking. All rights
reserved.
Instructors using the textbook may use and modify these slides for pedagogical purposes.
*AP is a registered trademark of The College Entrance Examination Board which was not involved in
the production of, and does not endorse, this product.
Arrays
 Arrays are objects that help us organize large
amounts of information
 Chapter 6 focuses on:
•
•
•
•
•
•
•
•
array declaration and use
passing arrays and array elements as parameters
arrays of objects
searching an array
sorting elements in an array
hashing
two-dimensional arrays
the ArrayList class
• polygons, polylines, and more button components
2
Arrays
 An array is an ordered list of values
Each value has a numeric index
The entire array
has a single name
0
scores
1
2
3
4
5
6
7
8
9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
This array holds 10 values that are indexed from 0 to 9
3
Arrays
 A particular value in an array is referenced using the
array name followed by the index in brackets
 For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
 That expression represents a place to store a single
integer and can be used wherever an integer variable
can be used
4
Arrays
 For example, an array element can be assigned a
value, printed, or used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
System.out.println ("Top = " + scores[5]);
5
Arrays
 The values held in an array are called array elements
 An array stores multiple values of the same type (the
element type)
 The element type can be a primitive type or an object
reference
 Therefore, we can create an array of integers, or an
array of characters, or an array of String objects,
etc.
 In Java, the array itself is an object
 Therefore the name of the array is a object reference
variable, and the array itself must be instantiated
6
Declaring Arrays
 The scores array could be declared as follows:
int[] scores = new int[10];
 The type of the variable scores is int[] (an array of
integers)
 Note that the type of the array does not specify its
size, but each object of that type has a specific size
 The reference variable scores is set to a new array
object that can hold 10 integers
 See BasicArray.java (page 300)
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Declaring Arrays
 Some examples of array declarations:
double[] prices = new double[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
8
Bounds Checking
 Once an array is created, it has a fixed size
 An index used in an array reference must specify a
valid element
 That is, the index value must be in bounds (0 to N-1)
 The Java interpreter throws an
ArrayIndexOutOfBoundsException if an array
index is out of bounds
 This is called automatic bounds checking
9
Bounds Checking
 For example, if the array codes can hold 100 values,
it can be indexed using only the numbers 0 to 99
 If count has the value 100, then the following
reference will cause an exception to be thrown:
System.out.println (codes[count]);
 It’s common to introduce off-by-one errors when
using arrays
problem
for (int index=0; index <= 100; index++)
codes[index] = index*50 + epsilon;
10
Bounds Checking
 Each array object has a public constant called
length that stores the size of the array
 It is referenced using the array name:
scores.length
 Note that length holds the number of elements, not
the largest index
 See ReverseOrder.java (page 302)
 See LetterCount.java (page 304)
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Initializer Lists
 An initializer list can be used to instantiate and
initialize an array in one step
 The values are delimited by braces and separated by
commas
 Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};
12
Initializer Lists
 Note that when an initializer list is used:
• the new operator is not used
• no size value is specified
 The size of the array is determined by the number of
items in the initializer list
 An initializer list can only be used only in the array
declaration
 See Primes.java (page 308)
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Arrays as Parameters
 An entire array can be passed as a parameter to a
method
 Like any other object, the reference to the array is
passed, making the formal and actual parameters
aliases of each other
 Changing an array element within the method
changes the original
 An array element can be passed to a method as well,
and follows the parameter passing rules of that
element's type
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Arrays of Objects
 The elements of an array can be object references
 The following declaration reserves space to store 25
references to String objects
String[] words = new String[25];
 It does NOT create the String objects themselves
 Each object stored in an array must be instantiated
separately
 See GradeRange.java (page 310)
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Command-Line Arguments
 The signature of the main method indicates that it
takes an array of String objects as a parameter
 These values come from command-line arguments
that are provided when the interpreter is invoked
 For example, the following invocation of the
interpreter passes an array of three String objects
into main:
> java StateEval pennsylvania texas arizona
 These strings are stored at indexes 0-2 of the
parameter
 See NameTag.java (page 311)
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Arrays of Objects
 Objects can have arrays as instance variables
 Many useful structures can be created with arrays
and objects
 The software designer must determine carefully an
organization of data and objects that makes sense
for the situation
 See Tunes.java (page 312)
 See CDCollection.java (page 314)
 See CD.java (page 316)
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Searching
 A common task when working with arrays is to
search an array for a particular element
 A linear or sequential search examines each element
of the array in turn until the desired element is found
 A binary search is more efficient than a linear search
but it can only be performed on an ordered list
 A binary search examines the middle element and
moves left if the desired element is less than the
middle, and right if the desired element is greater
 This process repeats until the desired element is
found
 See Searches.java (page 319)
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Sorting
 Sorting is the process of arranging a list of items in a
particular order
 The sorting process is based on specific value(s)
• sorting a list of test scores in ascending numeric order
• sorting a list of people alphabetically by last name
 There are many algorithms for sorting a list of items
 These algorithms vary in efficiency
 We will examine two specific algorithms:
• Selection Sort
• Insertion Sort
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Selection Sort
 The approach of Selection Sort:
• select a value and put it in its final place into the list
• repeat for all other values
 In more detail:
• find the smallest value in the list
• switch it with the value in the first position
• find the next smallest value in the list
• switch it with the value in the second position
• repeat until all values are in their proper places
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Selection Sort
 An example:
original:
smallest is
smallest is
smallest is
smallest is
1:
2:
3:
6:
3
1
1
1
1
9
9
2
2
2
6
6
6
3
3
1
3
3
6
6
2
2
9
9
9
 See SortGrades.java (page 323)
 See Sorts.java (page 324) -- the selectionSort
method
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Swapping
 Swapping is the process of exchanging two values
 Swapping requires three assignment statements
temp = first;
first = second;
second = temp;
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Insertion Sort
 The approach of Insertion Sort:
• pick any item and insert it into its proper place in a sorted
sublist
• repeat until all items have been inserted
 In more detail:
• consider the first item to be a sorted sublist (of one item)
• insert the second item into the sorted sublist, shifting the
first item as needed to make room to insert the new addition
• insert the third item into the sorted sublist (of two items),
shifting items as necessary
• repeat until all values are inserted into their proper positions
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Insertion Sort
 An example:
original:
insert 9:
insert 6:
insert 1:
insert 2:
3
3
3
1
1
9
9
6
3
2
6
6
9
6
3
1
1
1
9
6
2
2
2
2
9
 See Sorts.java (page 324) -- the insertionSort
method
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Sorting Objects
 Integers have an inherent order, but the ordering
criteria of a collection of objects must be defined
 Recall that a Java interface can be used as a type
name and guarantees that a particular class
implements particular methods
 We can use the Comparable interface and the
compareTo method to develop a generic sort for a set
of objects
 See SortPhoneList.java (page 328)
 See Contact.java (page 329)
 See Sorts.java (page 324) – the second
insertionSort method
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Comparing Sorts
 Time efficiency refers to how long it takes an
algorithm to run
 Space efficiency refers to the amount of space an
algorithm uses
 Algorithms are compared to each other by
expressing their efficiency in big-oh notation
 An efficiency of O(n) is better than O(n2), where n
refers to the size of the input
 Time efficiency O(2n) means that as the size of the
input increases, the running time increases
exponentially
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Comparing Sorts
 Both Selection and Insertion sorts are similar in
efficiency
 They both have outer loops that scan all elements,
and inner loops that compare the value of the outer
loop with almost all values in the list
 Approximately n2 number of comparisons are made
to sort a list of size n
 We therefore say that these sorts have efficiency
O(n2), or are of order n2
 Other sorts are more efficient: O(n log2 n)
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Hashing
 Hashing is a technique used to efficiently store and
retrieve data in an array
 An array used for hashing is called a hash table
 A hash function calculates a hash code for each data
item.
 The hash code is used as an index into the array,
telling where the data item should be stored
 Example: hash function f(n) = n % 7
• Element 18 would be stored in array cell 18 % 7 or 4
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Two-Dimensional Arrays
 A one-dimensional array stores a list of elements
 A two-dimensional array can be thought of as a table
of elements, with rows and columns
one
dimension
two
dimensions
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Two-Dimensional Arrays
 To be precise, a two-dimensional array in Java is an
array of arrays
 A two-dimensional array is declared by specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
 A two-dimensional array element is referenced using
two index values
value = scores[3][6]
 The array stored in one row or column can be
specified using one index
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Two-Dimensional Arrays
Expression
scores
Type
int[][]
Description
scores[5]
int[]
array of integers
scores[5][12]
int
integer
2D array of integers, or
array of integer arrays
 See TwoDArray.java (page 335)
 See SodaSurvey.java (page 336)
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The ArrayList Class
 The ArrayList class is part of the java.util package
 Like an array, it can store a list of values and reference
them with an index
 Unlike an array, an ArrayList object grows and shrinks
as needed
 Items can be inserted or removed with a single method
invocation
 It stores references to the Object class, which allows it to
store any kind of object
 See DestinysChild.java (page 339)
 See Recipe.java (page 341)
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ArrayList Efficiency
 The ArrayList class is implemented using an array
 The code of the ArrayList class automatically
expands the array's capacity to accommodate
additional elements
 The array is manipulated so that indexes remain
continuous as elements are added or removed
 If elements are added to and removed from the end of
the list, this processing is fairly efficient
 If elements are inserted and removed from the middle
of the list, the elements are constantly being shifted
around
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Polygons and Polylines
 Arrays often are helpful in graphics processing
 Polygons and polylines are shapes that can be
defined by values stored in arrays
 A polyline is similar to a polygon except that its
endpoints do not meet, and it cannot be filled
 See Rocket.java (page 359)
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The Rocket Program
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The Polygon Class
 The Polygon class, defined in the java.awt package
can be used to define and draw a polygon
 Two versions of the overloaded drawPolygon and
fillPolygon methods each take a single Polygon
object as a parameter
 A Polygon object encapsulates the coordinates of
the polygon
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Check Boxes
 A check box is a button that can be toggled on or off
 A check box is represented by the JCheckBox class
 A change of state generates an item event
 The ItemListener interface corresponds to item
events
 The itemStateChanged method of the listener
responds when a check box changes state
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The StyleOptions Program
 A frame is a container that can be used to create
stand-alone GUI applications
 A frame is represented by the JFrame class
 A Font object represents by the font's:
• family name (such as Times or Courier)
• style (bold, italic, or both)
• font size
 See StyleOptions.java (page 363)
 See StyleGUI.java (page 364)
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The StyleOptions Program
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Radio Buttons
 A set of radio buttons represents a set of mutually
exclusive options
 When a radio button from a group is selected, the
other button currently "on" in the group is toggled off
 A radio button generates an action event
 See QuoteOptions.java (page 366)
 See QuoteGUI.java (page 368)
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The QuoteOptions Program
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Summary
 Chapter 6 has focused on:
•
•
•
•
•
•
•
•
array declaration and use
passing arrays and array elements as parameters
arrays of objects
searching an array
sorting elements in an array
hashing
two-dimensional arrays
the ArrayList class
• polygons, polylines, and more button components
42

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